This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We have analyzed antigen-specific T cells directly ex vivo using peptide/MHC-tetramers and functional assays that measure cytolytic activity and cytokine production. Following acute viral infection, we have found that T cell responsiveness (termed functional avidity) to peptide antigen increased significantly in both normal and T cell receptor (TcR) transgenic mice, even though TcR avidity remained virtually unaltered. Little is known about the factors involved with determining the kinetics or degree of functional avidity maturation. In our recent studies, we have published a review describing T cell attributes and how they can be modified by adaptive (i.e., peptide stimulation) vs. innate (i.e., cytokine-mediated stimulation) mechanisms of activation. We have also determined the surface phenotype of T cells with high functional avidity. Prior studies had suggested that CTLA-4+ T cells had lower functional avidity than CTLA-4- T cells. Our data refutes this previous observation as we show that CD8+ T cells from LCMV-infected mice respond equally well against graded doses of specific peptide or graded doses of anti-CD3 stimulation, regardless of their expression of CTLA-4 molecules. This is significant because CTLA-4 is considered the predominant down-regulatory molecule on T cells and we show that this is not the case, at least in terms of antiviral T cell responses against either LCMV or vaccinia.